Collector for solar radiation

ABSTRACT

A solar collector arrangement includes a number of assemblies ( 1 ), which are immersed or partially immersed in a pond of water ( 2 ). Each assembly ( 1 ) includes a parabolic reflector ( 3 ) and an absorber ( 6 ). Barriers ( 10 ) are located on or near the surface of the water ( 2 ) and operate to reduce waves which may otherwise disturb the direct passage of sunlight in windy conditions. The complete immersion of the assembly ( 1 ) in the liquid serves to simultaneously protect and cool the apparatus, while allowing ease of sun 10  tracking movements by buoyancy induced rotation. Partially immersed versions have higher efficiency and protect against severe weather by inverting into the water.

TECHNICAL FIELD

The present invention relates to the tracking and protection of solarenergy collectors and the like.

BACKGROUND ART

There is a need to economically collect solar energy in concentratedform prior to direct use or conversion to electricity or other useableforms of energy. Solar energy has a modest intensity at the earth'ssurface of about 1000 Watts per square metre.

It is thus highly desirable to concentrate the energy to higherintensity (usually expressed in Watts per square metre, or W/m²) beforeuse. This is particularly so where solar cells are used to convert thesolar energy into electrical energy. The photovoltaic cells that areused to convert the solar energy into electrical energy are relativelyexpensive. Concentration of the incident solar energy into a smallerarea allows the use of a smaller area of energy conversion cells, withlower resulting costs of conversion cells. The key requirement in aconcentrating collector is a means to concentrate the energy as much aspossible with a system which is very low in cost per unit area and whichcan track the sun by rotation about one or two axes.

In the past, various forms of concentrator have been used. These haveincluded refractive concentrators (lenses) and, more commonly, curvedreflectors (mirrors). The concentrators are generally mounted onstructures that allow movement to follow or track the movement of thesun accurately across the sky each day. For economy the system used totrack the sun must be as simple and robust as possible. Present methodsof tracking use either motors and gears, or sliding hydraulic actuators,both of which add considerable cost. The need for tracking makes theconcentrator structures heavier and more complex than staticnon-concentrating solar energy collectors, because the trackingmovements usually require that all support be provided through rotaryjoints which are subject to very high forces during extreme winds.

Any structures used for concentration must be well protected from highwinds, hail and other aspects of extreme weather conditions. It is alsomost commonly desirable to provide some form of cooling of the devicesthat convert the concentrated solar energy to electricity.

Silicon photovoltaic cells, which are the most economical variety atthis point in time, operate less efficiently as their temperatureincreases. If a mechanism for cooling the cells is not used, the use ofconcentrators tends to cause the cells to operate at higher temperature,decreasing their energy conversion efficiency.

Most concentrators designed so far employ very substantial mechanicalstructures to resist movement and damage from the wind. In addition theyusually employ heavy and strong materials such as glass with metalbacking for the reflective element to protect the device from damage bywind, ice and hail. Such structures are, at present, either veryexpensive or too fragile for continuous outdoor use.

One method that has been used to protect the reflective concentratorsurface has been the use of an inflatable, aluminised, flexible plasticmembrane as a concentrating reflector. The shape of the membrane ismaintained by an air pressure difference from one side to the other.Such reflectors can be deflated during severe weather. They arerelatively cheap, but are still subject to damage from high winds and byultraviolet light. In addition they require substantial structures tosupport the moving parts against high winds.

Another method that has been used to protect concentrators is the use ofa transparent dome or building to cover and protect the whole solarconcentrator. This does allow some simplification of the structuraldesign of the moving concentrator. However, this method has little or nooverall cost advantage due to the added cost of the protectivestructure.

The Yeomans patents WO93/09390 and U.S. Pat. No. 6,220,241 B1 usetemporary immersion in water to protect a reflective concentrator. Itconsists of a reflective concentrator floating on water with a heatcollector at the focal point in the air above. The concentrator can besubmerged using pumps for limited periods to avoid damage to theconcentrator mirror during adverse weather conditions. This is achievedby flooding its' buoyancy tanks with water, a change in the absolutebuoyancy of the apparatus. It is not able to operate as a solarconcentrator or energy collector while submerged. The concentrator maystill be damaged in bad weather if the mechanism fails or loses power ata critical time (it is not passively robust). This system also achievestracking movement in an azimuth direction (rotation about the verticalaxis only), using movement within the water. Tracking about a horizontalaxis is achieved via motors, gears and levers.

Virtually all the existing concentrating collectors require themechanism to move to a special protected position for protection againstadverse weather, making them particularly vulnerable to damage whenthere is a mechanical or electrical breakdown.

Russian patent number SU1430-927-A to Novorossiisk Naval describes thegeneral concept of floating a flexible transparent sack in water tocreate a lens, but no details are given of the material to fill thissack, or of any energy collector or conversion device, or of the scaleof the device and no tracking method is proposed.

Aims

The present invention accordingly aims to provide protection againstweather conditions and ultraviolet radiation for solar concentrators andcollectors. Subsidiary aims of the present invention are, to provide asimple means for tracking solar concentrators to follow the sun and toprovide cooling for solar collectors, and to provide more lightweightstructures than was previously possible. The invention addresses theseaims, at least in part, by using the protective, cooling and buoyancyproperties of a body of liquid, such as a lagoon, pond, tank, lake, damor the like of water or other liquid.

SUMMARY OF THE INVENTION

The present invention accordingly provides a collector for thecollection of solar radiation, which collector includes:

at least one energy conversion device for the conversion ofelectromagnetic energy into another form of energy; and

at least one concentration device for the reception of electromagneticenergy and the concentration of it onto the energy conversion device inwhich both devices are at least partially immersed during operationwithin the same body of liquid.

It is preferred that the liquid is water.

In one preferred form of the collector, the conversion device includes aphotovoltaic cell. In this embodiment of the invention, the photovoltaiccell is encased in a hermetic seal.

In an alternative preferred form of the invention, the energy conversiondevice includes a component that converts incident electromagneticenergy into heat; and which is enclosed by a vacuum chamber.

In an alternative preferred form of the invention, the energy conversiondevice includes a component which converts incident electromagneticenergy into stored chemical energy via a photochemical reactor usingTitanium Dioxide or other photo-catalyst which may split water intohydrogen and oxygen or enhance other useful chemical processes.

One preferred form of the concentration device includes a mirror. Analternative preferred form of concentration device includes a lens.

It is preferred that the concentration device is fabricated at least inpart from plastics material.

It is preferred that the collector be provided with at least twointerconnected and vertically extended lateral buoyancy tanks onopposite sides with the total buoyancy being set constant and sufficientto keep the whole unit substantially below the water surface, but withpositive buoyancy so that the unit floats just below the surface. It ispreferred that the relative buoyancy of these two tanks should beadjusted by interchange of liquid and air between them to cause thecollector to rotate about the horizontal axis perpendicular to the linebetween the two tanks to provide simple tracking of the direction ofincoming solar radiation. These two interconnected buoyancy tanks can beimplemented in the form of a curved tube whose sealed ends are below thewater surface with the curve of the tube rising above the surface. Usingthis method the net or total buoyancy of the apparatus does not change.The total buoyancy is kept constant, with the tanks sealed to theoutside air and water, but the relative buoyancy of the pair isadjustable. In this embodiment the concentrator is suspended under thewater from floats at the surface, so that there is no need for asubstantial support structure mounted at the bottom of the pond, and noneed to control the depth of the water precisely, and no need for anybearings or rotating joints.

It is preferred that the collector includes means to inhibit theformation of waves at the interface between the body of liquid and theair. It is especially preferred that the means for inhibiting theformation of waves is substantially transparent to the solar radiation.

It is preferred that the liquid includes a component or additive tosuppress the growth of algae and bacterial slime.

The invention accordingly addresses the aims of the invention, at leastin part, by using the protective and buoyancy properties of a body ofliquid, such as the sea, a pond, tank, dam, lake or the like of water orthe like.

There are two principal varieties of the invention, the first being thatwhere both concentrator and energy converter are permanently immersed,as illustrated in FIGS. 1 and 3 and 4, while the second principalvariety, as illustrated in FIGS. 5 and 6, is that where the energyconverter is substantially immersed during operation, but theconcentrator is only partially immersed for part of the operating day,and may be fully immersed by rotating the tracking system to point theconcentrator downwards.

The advantages of the first principal variety are that the continuousimmersion of the concentrator passively protects it against weather atall times, that immersion allows the use of a simple lateral buoyancybalance to achieve tracking, and that PV cells are naturally cooled byimmersion in a body of water.

The advantages of the second principal variety are that immersion allowsprotection of the concentrator when required by rotation of the buoyancytracking system, and that partial immersion allows the use of a simplelateral buoyancy balance to achieve tracking of a short focal lengthsystem (which is the lightest and most stable system), and that PV cellsare naturally cooled by immersion in a body of water. In addition, thesecond variety has little water in the optical path so that it producesmore energy per unit area.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a portion of apparatus according to oneembodiment of the present invention; and

FIG. 2 is a chart illustrating the efficiency of light utilisation ofone embodiment of the present invention; and

FIG. 3 is an elevation view of a portion of apparatus according toanother embodiment of the present invention, similar to that of FIG. 1;and

FIG. 4 is a perspective view of another embodiment of the presentinvention, using a point focus concentrator; and

FIGS. 5 and 6 are elevation views of other embodiments of the presentinvention, wherein the concentrator is substantially above the waterlevel during part of the operational cycle each day.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

According to the embodiment of the present invention which isillustrated by reference to FIG. 1, a plurality of assemblies, one ofwhich is generally indicted by reference numeral 1, are immersed in apond or other body of transparent liquid 2. It is preferred that thepond of liquid 2 be water.

The complete immersion of the assembly 1 in the liquid serves tosimultaneously protect and cool the apparatus whilst allowing easyrotation about any axis through the center of gravity of the collector.The assembly is given slight positive buoyancy to keep it substantiallybelow the surface, but touching the surface.

The solar collector assembly includes a reflector that is generallyindicated at 3 (shaded in the diagram). In its preferred form, thereflector 3 has a reflecting surface which is substantially parabolic ina cross-section of the collector, and which extends longitudinally toform a generally trough-shaped reflecting surface.

Each reflector assembly 3 is constructed on a base of thin, rigidplastics material (preferably acrylic or polycarbonate) to which isadded a metallic reflective layer. The reflective layer is then sealedfrom liquids with a transparent plastics cover layer such aspolypropylene, acrylic, Mylar or other suitable material. Preferredforms of reflector material include aluminium, silver and rhodium. Theparabolic shape is maintained by attaching a plurality of onedimensional parabolic formers 4 and straight interconnecting stringers 5perpendicular to the rear of the reflective surface.

In one arrangement of the assembly shown in FIG. 1, the long axis ofeach trough reflector 3 runs in a generally North-South direction if thesystem is using east-west tracking (horizontal axis tracking). Alinearly extending array of photovoltaic cells 6 is mounted at the focusof the reflector assembly 3, accordingly similarly running in aNorth-South direction. This North-South orientation is best suited totropical regions with high sun angles all year. An alternativearrangement that better suits higher latitudes is North-South trackingof trough reflectors with their long axes aligned East-West. To allowhorizontal axis (azumithal) sun tracking, the reflector assembly 3 isprovided with a sealed buoyancy tank in the shape of a curved tube 7,which passes above the height of the photovoltaic cells 6. The buoyancytank can be placed at one end of the assembly to avoid shading, withanother buoyancy tank added to the other end of the assembly to providebalanced support. The total buoyancy of the assembly can be adjusted byadding a suitable amount of water or other fluid such as Ethylene Glycolto each of the tanks to give the whole assembly slight positivebuoyancy, sufficient to bring the top edge of the curved buoyancy tubeabove the water while keeping the photovoltaic cells 6 just below thewater surface 2. The remainder of each buoyancy tank and connecting tubeis filled with air. Under these conditions the assembly can be rotatedin the water about its' center of gravity by changing the relativelateral buoyancy without altering the total. This rotation is achievedby pumping small amounts of fluid from the right to left tank orvica-versa using very small sealed electrical pumps mounted within eachbuoyancy tank at 8 and 9. The delivery side of each pump is connected bya small tube (which is not shown in the diagram), to the opposite end ofthe curved tube 7 thus allowing movement of water to the opposite endwhen required. A single, reversible positive-displacement pump can beused as an alternative, as illustrated in FIG. 3. The pumps are switchedby a simple automatic sun-tracking circuit controlled by a pair ofphotosensitive cells, which are mounted on each side of a shadow vane onthe north-south focal axis of the system, thus aligning the wholeassembly with the sun's rays. Such electronic servomechanisms are wellknown and not shown here. This rotational movement of the reflectorassembly 3 about the center of gravity of the assembly allows trackingof the sun from East to West during the day. Since the rate of movementrequired is slow (less than 15° per hour) and the assembly is notexposed to wind, there are no significant forces acting on the assemblyof FIG. 1 except the slight drag of the water. The rate of movement ofthe sun is 15° per hour, but the increase in refractive index that isencountered by the sunlight in entering the water reduces the actualangular rate of movement required in the collector slightly. Ifrequired, tracking can be achieved on two axes by the use of a secondpair of buoyancy tanks at right angles to the first pair. In thisembodiment the concentrator is suspended under the water from the airfilled section of the buoyancy tracking tubes 7 at the surface, so thatthere is no need for a substantial support structure mounted at thebottom of the pond, and no need to control the depth of the waterprecisely.

A higher cost alternative (not shown in this illustration) which givesmaximum performance at any latitude, is a system which provides two-axistracking by allowing rotation around a vertical axis together withazimuthal tilt (horizontal axis tracking). Rotation about the verticalaxis can be achieved through motor-driven propellers at a tangent to acircle in the plane of rotation or through a motor-driven paddle wheelwith a vertical axis of rotation to provide horizontal tangentialthrust. Such two axis tracking systems can use two dimensional (dish)concentrators.

When the concentrator system is fully immersed in water it is no longernecessary to employ strong or heavy materials for the reflector as themovements and pressures caused by wind and weather reduce very rapidlywith depth in water. This allows the use of relatively lightweightstructural materials such as plastics for almost all parts of thecollector. In addition, the structural deflections caused by gravity aregreatly reduced, since the densities of typical plastics employed in thestructure are only around twenty percent higher than the surroundingwater. Components are protected from most forms of environmental damageby immersion in water, including protection from high winds, hail,windblown dust and short wavelength UV light. Ultraviolet light damagesmany plastics. However, with the fully immersed form of the presentinvention, wavelengths below about 250 nanometres are filtered out bypassage of the sunlight through water, so long as the passage throughwater is longer than about 50 cm. This allows longer-term use of cheaperforms of plastics materials, which would otherwise not be suitable foruse when exposed directly to sunlight.

A linearly extending array of energy absorbing devices 6, preferablyphotovoltaic (PV) cells, is located at the focus of the reflectorassembly 3, and is mounted to move in synchronism with the reflectorassembly 3. It is preferred that the energy absorbing assembly 6 ismounted in place at the focus by a support assembly 11 (such as of clearplastics material) that mounts directly on the reflector assembly 3.When photovoltaic cells are used as energy absorbing devices, they areencased in a thin hermetic seal (preferably of a suitably transparentplastics material, such as Tedlar, or of glass) to prevent water damageto the semiconductor. This encasing material however must be of adequateconductivity to heat, to allow the cells to be cooled by the surroundingwater. Any space remaining between the encasing material and thephotovoltaic cells should be filled with a transparent non-corrosiveliquid such as Silicone oil or with a transparent flexible solid such assilicone rubber. The surrounding water provides convective liquidcooling of the energy conversion device placed at the focus of theconcentrator. According to further preferred embodiments of theinvention (which are illustrated in FIG. 6), the photovoltaic cells orthe like are mounted on a heat-conducting substrate. Preferred materialsfor this mounting substrate include copper, aluminium and aluminaceramics. It is especially preferred that that this mounting substratealso be encased with a thin layer of suitable plastics material (such asTedlar) to form a hermetic seal if the surrounding liquid is corrosiveto the substrate. Portions of the mounting substrate remote from thecells are in contact with the liquid, enhancing cooling of the cells. Ifnecessary in a particular installation, further means are provided forenhancing transfer of heat from the substrate to the liquid. Preferredmeans of enhancing heat transfer include heat sinking fins attached toor integral with the substrate, and channels through the substrate whichare in communication with the surrounding liquid.

If the sun's energy is to be absorbed as heat or for the purpose ofdriving a chemical process, rather than converted to electricity byphotovoltaic cells, it is preferred that a transparent cylindricalvacuum chamber is provided at the reflector focus, surrounding theenergy converter 6, to prevent water cooling of the absorber.

The depth of the liquid 2 in FIG. 1 that is required depends on thechosen width of the focal absorbing strip (PV cells), which is typically10 mm to 50 mm. The reflector concentration ratio is typically 20 to 50,and this requires a trough width of about 1 m to 5 m for a 50 mm strip,or 200 mm to 500 mm for a 10 mm wide focal strip.

A parabolic reflector concentrator with a flat absorber generally needsa focal length of approximately one half of the aperture or a littlemore, so the focal length is in the range of 150 mm to 2.5 m. Thus theminimum water depth would be between 200 mm and 3 m, typically 1 m. Thusthe reflector is typically one metre wide with a 20 mm wide focal strip,in a depth of water of about one metre.

There is significant attenuation of the longer wavelengths of thesunlight by the water. Experiments at one metre of optical path in clearwater have determined that this results in a reduction of output fromthe present silicon photovoltaic cells to around 45% to 50% of theirfully exposed levels. To compensate for this effect it is necessary touse larger reflector areas, but this does not greatly increase theoverall capital cost since the reflector can be made of lightweight, lowcost materials. If the PV cells are designed for optimal spectralresponse for the underwater application, these losses may be reduced. Asuitable form of PV cell is that made from Indium Gallium Phosphide,which is very efficient in the visible light range using wavelengthsfrom 400 to 700 nm. In addition, a shorter optical path length in thewater will reduce losses, so the assembly should be kept as near thesurface as possible.

The cost of the pond required for immersion of the collector assemblies1 is not a large factor, since it is similar in structure to low costwater retaining dams, or may even be a natural pond or salt lake or asaltwater inlet or lagoon connected to the sea. Large arrays ofcollector assemblies can be employed in a single pond covering manythousands of square metres.

It is preferred that the pond of water is provided with regularly spacedfixed and/or floating barriers 10 (which are preferably transparent whenthey intercept light travelling to the concentrators). It is preferredthat the refractive index of the barriers be close to that of the water,to minimise losses. These barriers are located on or near the surfaceand operate to reduce waves that may otherwise disturb the directpassage of sunlight in windy conditions. At least one such barrier 10 isplaced between adjacent parallel rows of concentrator assemblies 1 witha typical spacing of 1.5 m. Cross-rows of barriers (which are notillustrated in the drawings) are also preferably placed at regularintervals at right angles to the rows of concentrators 1. It ispreferred that the spacing between these cross-rows is about three tosix metres.

Alternatively, or in addition, a thin layer of clear mineral oil orother suitable high viscosity transparent liquid can be provided tofloat on the surface of a lower liquid to reduce the build-up of surfacewaves.

Alternatively, or in addition, a thin transparent membrane orsmall-scale cell structure can be provided to float on the surface toreduce the build-up of surface waves.

To avoid the build up of algae and other organic contaminants oncritical surfaces, the water of the pond preferably contains a suitablecomponent or additive to suppress growth of algae. It is preferred thatthis component or additive be common sodium chloride (at ‘Dead Sea’levels), other salt or other transparent chemical additive which killsalgae used alone or in combination. Certain such additives, includingsodium chloride, can additionally aid against freezing of the water.Other preferred additives to suppress algae growth include copper basedalgaecides, chlorination, and ozone or ultraviolet treatment of thewater. Alternatively, or in addition, moving mechanical cleaners can beemployed, which remove algae and bacteria with high velocity water jets,or suitable water snails, fish or other organisms may be employed to eatthe contaminants.

Alternatively, algae and bacteria can be suppressed by raising thetemperature of the pond water sufficiently high to kill such organismsperiodically. This may be achieved by the incoming solar radiation alonewhen a transparent surface membrane is employed to cover nearly thewhole pond.

FIG. 2 serves to illustrate the relative power available from a siliconphotovoltaic cell after surface reflections at the upper surface of thewater and passage of sunlight through two metres of water to the cell.This is an overall throughput of about 50% of the sunlight energyincident on the upper surface of the water which would normally beavailable to a Silicon PV cell.

Making the reflector assembly 3 of the concentrator larger readilycompensates for the loss of efficiency caused by passage of the lightthrough water. Since the cost of the reflector per unit area is very low(being made of lightweight plastics) compared to the cost ofphotovoltaic cells, this is not a significant cost. The area ofphotovoltaic cells required for a given power output is not changed bylosses in transmission through water, but the reflector area must beincreased relative to the area required for a normal concentratingcollector in the air.

According to the alternative embodiment of the present invention whichis illustrated by reference to FIG. 3, an assembly similar to FIG. 1 isshow in elevation, or end view, substantially immersed in a body ofwater 2. The reference numbers used here and in subsequent Figures matchthose of FIG. 1 for corresponding elements. In this embodiment thephotovoltaic cells 6 are kept close to the centre of rotation 17 andtanks 15 and 16 are used to prevent the reflector 3 from rising abovethe water surface at the most extreme rotations, and a single pump 8 isemployed. In this embodiment a sealed hollow buoyancy tracking tube 7,approximately half filled with liquid 18, is employed with reversible,bidirectional positive displacement pump 8, to rotate the assembly asone unit about the centre of rotation 17 of the tracking tube loop 7.The preferred liquid in 7 is ethylene glycol or water and the remainingspace in the tube is filled with air. The rotation is effected bymovement of the liquid enclosed within 7 from left to right through thepump 8, or visa-versa, to change the left-right balance of the systemvia the resulting shift in the buoyant zones 12 and 13. The pump 8 isdriven by an electric motor that is controlled in speed and direction byan electronic servo mechanism fed by left and right light sensorsaligned with the vertical axis of the system. Such servomechanisms arewell known and so are not illustrated here.

Tanks 15 and 16 are sealed tubes running the full length of the edges ofthe reflector which are thin walled and filled with water to be ofapproximately neutral buoyancy when submerged. At extreme tilt to theleft (counterclockwise), tank 16 will rise to the surface. As 16 breaksthe surface the mass of water contained in 16 tends to prevent thecorresponding reflector edge from rising out of the water. Instead theremainder of the assembly will move deeper into the water as the systemrotates further counterclockwise, allowing tracking of the sun to moreextreme angles while maintaining energy collection from the wholesurface of the reflector 3. A similar action occurs on clockwisemovement when tank 15 rises to the surface. This embodiment, using 15and 16, allows the reflector to be kept, on average, closer to thesurface than would otherwise be possible, thus reducing the losses dueto the length of the optical path through the water medium.

According to the alternative embodiment of the present invention whichis illustrated in perspective by reference to FIG. 4, the apparatus usesa point focus or two dimensional parabolic concentrator 3 with a pair ofnear circular buoyancy tracking tube loops 7 and 17 mounted verticallyand at right angles to each other to provide full tracking of the sun intwo dimensions. An array of photovoltaic cells 6 is mounted near thefocus of the substantially parabolic reflecting concentrator 3. Thecomponents are mounted to move as one unit. The assembly is keptsubstantially underwater by adding sufficient liquid 18 to sealed tubes7 and 17, the balance of the upper section of each tube being filledwith air or inert gas. The preferred liquid 18 is ethylene glycol orwater. The lowest point of each of tubes 7 and 17 contains a positivedisplacement pump 8 or 9 controlling movement of liquid between thelower sections of each of the tubes 7 and 17. There is no communicationof liquid between the tubes 7 and 17 and each tube is sealed. Thus therelative lateral buoyancy of each of the tubes can be adjusted by pump 8and 9 to rotate each tube in relation the water surface about an axisperpendicular to the plane of each tube. These pumps are controlled by apair of photocells and servomechanism for each axis, as described forthe earlier embodiments. Since the tracking system will maintainpointing of the assembly at the brightest point in the sky at all times,it is not necessary to orient the system to North/South. To keep theassembly in one location and provide a path for the power wiring, it isdesirable to have a flexible anchor rope and anchorage 22 to the bottomof the pond. A typical size for the round parabolic reflector 3 in thissystem is about one metre diameter.

The embodiment shown in FIG. 4 has advantages over the method of FIG. 1in that this embodiment uses two dimensional concentration which allowslower focussing accuracy on each axis for a given level ofconcentration. Thus it is more tolerant of waves and ripples and lessprecise manufacturing methods may be required for the reflector 3.

FIGS. 5 and 6 are preferred embodiments of the second principal varietyof the invention where the concentrator is not permanently immersed, butmoves partially into the water during tracking rotations and fully intothe water when rotated 180 degrees from the zenith.

According to the alternative embodiment of the present invention whichis illustrated in plan view by reference to FIG. 5, the apparatus uses arefractive Fresnel lens 3, preferably made of transparent plasticsmaterial, joined as shown to a buoyancy tracking loop tube 7 (or a pairof such tubes at right angles for a two dimensional trackingconcentrator, similar to that in FIG. 4) and Photovoltaic energyconverters 6, similar to those described for previous embodiments. Thenumbers used match those of previous Figures for correspondingcomponents. In this embodiment the apparatus is only partiallysubmerged, such that, while the PV cells 6 are always submerged, theFresnel lens 3 is above the water surface 2 for about half of thetracking tube 7 rotation. FIG. 5 shows the apparatus oriented to pointabout 30 degrees above the horizon. As previously described, thetracking tube 7 contains a positive displacement pump 8 which movesfluid 18 within the lower section of tube 7 to set the position ofbuoyant zones 12 and 13 which position the rotation of the collectorsystem to track the angle of the sun's rays (which enter along the axisindicated by the arrow). During extreme weather conditions extraprotection can be provided by providing a further positive displacementpump 9 within tube 7 at a suitable position as shown to allow thetracking system to almost fully invert the collector, immersing theconcentrator and thus greatly reducing the area of the exposed sectionand reducing wind loads. The pumps 8 and 9 are spaced approximately 120degrees apart. With the components positioned as shown in FIG. 5 thecollector is able to track the sun over at least 120 degrees,corresponding to 8 hours of the sun's movement. A third pump 120 degreesaway from 8 and 9 can be optionally employed to provide full rotationalcapability. Component 19 is a slightly conical tube whose inner surfaceis highly reflective (Aluminised), channeling light from lens 3 to thePV cells 6 and acting as a secondary concentrator which further focusesthe light. Tube 19 can also act to even out the variations in thefocussed light by multiple reflections, allowing more efficientoperation of the PV cells 6. The walls of 19, together with transparentwindow 20 and enclosure of PV cells 6, make a sealed enclosure whichprevents dirt or water contamination of the PV cells. The walls of 19may be a conductive metal such as copper to assist in removing heat fromthe PV cells 6 to the surrounding water and a metallic heat spreader 25may be attached to the rear of the PV cells. This enclosure mayoptionally be filled with a transparent liquid such as hydrocarbon oilfor better cooling. The Fresnel Lens 3 should have its' grooved surfacefacing the PV cells to minimise dirt accumulation. The rear surface ofthe PV cells enclosure 6 is in contact with the water to providecooling. The tube 19 is attached to the tracking tube 7 and lens 3 bystruts 21 so that the whole assembly moves as one unit. The assemblyshould preferably be retained by a sliding ring looped around tube 7which in turn is attached to a fixture or weight at the bottom of thepond by a rope or elastic cord, which may also carry the power outputwires. One or more vanes may be attached radially outward from tube 7 inthe vicinity of pump 8 to reduce oscillation of the system caused bywind or waves. Surface wave-breaking barriers 10 should be employed asreferenced in FIG. 1. The water of the pond or reservoir need not bekept clean or filtered as it is not in long term contact with theoptical surfaces of 3 and 20 and no water enters the optical path duringoperation except during extreme tilt. In this embodiment a lesser degreeof protection is provided to the system against wind forces than that ofFIGS. 1 and 3, but there is a large reduction in energy losses as thereis no passage of light through water in this embodiment, so that thepower available per unit area of collector is higher. In addition thissystem generates more consistent power during each day than that of FIG.1 since there is no reduction or spreading of energy by refractionthrough and reflection from a water surface. There is some loss ofuseful collection area when the concentrator edges enter the water, butthe loss from this cause is a small percentage of the daily total energyproduction. When using a two-dimensional concentrator this embodiment ismore tolerant of waves and ripples and less precise manufacturingmethods may be required for the lens 3 than when implemented with a onedimensional concentrating lens.

According to the alternative embodiment of the present invention whichis illustrated in plan view by reference to FIG. 6, the apparatus uses areflective concave parabolic concentrator 3 combined with a smallerreflective convex secondary reflector 23 placed on axis just inside thefocus position of reflector 3. Secondary reflector 23 has a focal lengththat diverges the solar rays sufficiently to bring them to a focus inthe vicinity of the PV energy converter 6 through a window 20 at thecentre of reflector 3. The PV cells 6 are mounted in a position suchthat they are always substantially submerged below the pond water levelwhen the system is within about 60 degrees of vertical. The apparatus isonly partially submerged so that concentrator 3 remains above thesurface at most angles of operation. Conical tube 19 links PV cells 6and window 20 to form a sealed enclosure which prevents watercontamination of the PV cells. This enclosure may optionally be filledwith a non-corrosive transparent liquid to improve heat removal from thecells 6. A metallic heat spreader 25 may be provided at the rear of 6 tobetter conduct heat to the surrounding water. The inner walls of 19 arepreferably highly reflective internally to further concentrate the rays.As for FIG. 5, the system as shown may be a linear, one dimensionalconcentrator, or it may be a two dimensional concentrator and trackingsystem similar to that illustrated in FIG. 4 with the addition of asecond tacking tube at right angles to 7. In the one dimensional casetube 19 becomes a trough extending out of the plane of the diagram. Theapparatus floats on a pond containing wave suppressors, as for FIG. 5.Tracking tubes and pumps 8 and 9 operate as for FIG. 5, allowing suntracking and full inversion of the apparatus to submerge the largereflector 3 for protection in severe winds. A few small holes areprovided in 3 near 20 to allow drainage. A vane 24 is provided for eachaxis to reduce oscillation due to waves and wind forces. Vane 24 is afan-shaped concertina arranged on slide-locking hinges at its' tip tofold out of the wind when it is in an exposed position (inverted). Allcomponents are rigidly mounted together to rotate as one unit. All largecomponents are preferably made of thin plastics material. The apparatusis tethered to the pond bottom in a manner similar to that of FIG. 4.The advantages of this system over that of FIG. 5 are that only smoothsurfaces are required in the concentrator so that it is more readilycleaned by water spray and that the reflector can be more economicallymade with high strength due to its' parabolic shape. In other respectsthe system of FIG. 6 is similar in performance to that of FIG. 5. Thewater of the pond or reservoir need not be kept clean or filtered as itis not in long term contact with the optical surfaces of 3 and 23 and nowater enters the optical path during operation except briefly duringextreme tilt, so dirt in the water is of little consequence. Theembodiments of FIGS. 5 and 6 both require that the mass of the apparatusbe balanced approximately around the centre of rotation 17. For ease ofbalance and a low wind profile it is necessary that the lens of FIG. 5and the reflector of FIG. 6 be kept as close as feasible to the watersurface. This requirement causes the edges of the concentrator devices 3of FIGS. 5 and 6 to become partially immersed at the ends of the dailytracking cycle when the pointing direction is close to the horizon.Windbreaks may be employed above the water surface to reduce oscillationof these systems.

The bi-directional positive displacement pumps 8 and 9 shown in FIGS. 3,4, 5 and 6 include an electric motor drive and either gear or vane orperistaltic pump, or they may be implemented in the form of pulsed pumpsusing solenoids to compress flexible chambers. Such pumps are well knownso are not detailed here. These pumps need only be of very small size,power and capacity, typically moving one cubic centimeter per second.According to alternative preferred embodiments of the invention that arenot illustrated in the drawings, the apparatus uses underwaterconcentrators that are in the form of refractive lenses, optionally inFresnel, or segmented form. These preferably use transparent plasticsmaterials which are located near the water surface to form one or moreair filled voids under the water to focus light, and an energyconverting device such as a strip of photovoltaic cells which is locatedbelow the lens at the focal point. These embodiments have the advantagethat no metallic reflective layer is required, thus extending thepotential lifetime of the device. In such embodiments the concentratormay be suspended under the water attached to a floating buoyancytracking system as described for FIG. 3, so that there is no need for asubstantial support structure mounted at the bottom of the pond, and noneed to control the depth of the water precisely.

Yet further preferred embodiments of the invention which are notillustrated use holographic concentrators.

In any of the preferred embodiments described, the photovoltaic cellenergy converter 6 can be replaced either by a chemical reaction chamberoptionally containing a catalyst, or by a thermoelectric energyconverter

The present invention allows the use of very low-cost materials for thereflector assemblies 3 of FIGS. 1 and 3 and 4 and 5 and 6. Any apparentloss in overall efficiency due to transmission losses through the wateror other liquid is counterbalanced by providing larger reflectorassemblies 3 than would be used if the same photovoltaic cells were usedin a land-based collector assembly. This increase in size of reflectorassembly (or lens assembly) results in a higher incident light energydensity at the photovoltaic cells. Increasing the incident energyintensity on photovoltaic cells would normally lead to increased heatingof the cells, which is undesirable for a number of reasons. One of theseis that the efficiency of energy conversion of photovoltaic cells dropsoff as their temperature increases. However, having the photovoltaiccells within water provides natural convective cooling, allowingoperation of the cells at 50 times or more of the intensity of normalsunlight with only a small temperature increase on the cells.

Placing the apparatus underwater also provides some protection againstultraviolet light, because wavelengths below 250 nanometres are filteredout of the light by passage through water (if over about 500 mm path).This protection from ultraviolet light allows relatively long term useof cheaper forms of plastics materials, which would otherwise not besuitable when exposed to the sun.

The apparatus is placed in this situation underwater, or partiallyunderwater, to achieve five main objectives, which are:

-   -   1. Underwater placement reduces the disturbing effects of wind        on the focusing collector. These effects include distortion of        the structure by wind forces, which reduces the degree of        concentration possible. These effects also include disturbances        to the rotary tracking movements caused by wind forces.    -   2. Underwater placement greatly simplifies the requirements for        a tracking mechanism to keep the concentrator and collector        focused on the incoming radiation in that only changes in        relative angular (or lateral) buoyancy are required to create        precise and stable rotational movements. Tracking on any        horizontal axis can be achieved simply by moving a mass of water        from a partially filled, vertically extended buoyancy tank on        one side to similar tank on the opposite side (changing the        relative buoyancy in a closed system). Tracking about a vertical        axis, if required, can be achieved with small propeller driven        thrusters placed tangentially at the edges of the concentrator.    -   3. The placement of the energy converter underwater also        provides efficient convective cooling of the energy-collecting        device when required, especially when this collector is a set of        PV cells or thermoelectric converters (whose rear, cold junction        requires cooling). Both these energy collectors /converters        operate with greater efficiency when cooled, especially if        cooled below daytime ambient temperature, which is generally the        case when the cells are in a large body of open water.    -   4. The underwater situation provides structural components with        support via their buoyancy in water and reduces deflections        caused by wind so that components of much lower mass, strength        and cost may be utilized relative to such a concentrator used in        an exposed position.    -   5. A permanent underwater location during operation greatly        reduces all weather-related damage risk, including that from        hail and wind at all times with no active control or power        required. Such a system is passively robust.

It is believed that the use of lightweight materials will, in turn,reduce the costs of transport of materials to their site of installationand the handling costs associated with installation.

This invention is well suited to application in a hydroelectricpumped-storage system consisting of two dams at different altitudes witha motor-generator linked to a turbine in a tube between the two dams.The floating solar electric generators can supply energy to lift waterfrom the lower dam to the upper dam when sunlight is available thusstoring the energy. The floating solar collectors can cover most of thedam surface and can be arranged to have no contact with the bottom ofthe dam, using above-water tethers and wiring if needed to allowadaptation to widely varying water levels. The use of existing dams andreservoirs for solar energy collection, utilising the present invention,eliminates a large fraction of the usual site and set-up costs of asolar power plant.

1. A collector for the collection of solar radiation, which collectorincludes: a. a tracking mechanism to keep the collector pointing at thesun; b. at least one energy conversion device for the conversion ofelectromagnetic energy into either electrical energy or chemical energy;and c. at least one concentration device for the reception ofelectromagnetic energy and the concentration of the electromagneticenergy onto the energy conversion device, wherein (1) the energyconversion device is substantially immersed in liquid during itsoperation, (2) the concentration device is at least partially immersedin liquid during part of the daily operation cycle while collectingsolar energy, and (3) both devices being are immersed within the samebody of liquid.
 2. A collector as claimed in claim 1, in which theliquid is substantially transparent to visible light.
 3. A collector asclaimed in claim 1, in which the liquid is water.
 4. A collector asclaimed in claim 1, in which the liquid is a hydrocarbon.
 5. A collectoras claimed in claim 1, in which the conversion device includes aphotovoltaic cell.
 6. A collector as claimed in claim 1, in which theenergy conversion device includes a chemical reaction chamber exposed tothe concentrated electromagnetic energy.
 7. A collector as claimed inclaim 1, in which the energy conversion device includes a thermoelectricconverter.
 8. A collector as claimed in claim 1, in which the energyconversion device is encased in a hermetic seal.
 9. A collector asclaimed in claim 1, in which the concentration device includes a mirror.10. A collector as claimed in claim 1, in which the concentration deviceincludes a refractive lens.
 11. A collector as claimed in claim 1wherein the tracking mechanism includes motor-driven mechanicallinkages.
 12. A collector as claimed in claim 1 wherein the trackingmechanism includes lateral buoyancy tanks having adjustable buoyancyrelative to each other.
 13. A collector as claimed in claim 12 whereineach of the lateral buoyancy tanks extend vertically above the surfaceof the liquid and are interconnected at their highest portion by apassage.
 14. A collector as claimed in claim 12 wherein the lateralbuoyancy tanks are interconnected at their lowest portion by a passage,with a reversible positive displacement pump positioned in said passageto transfer liquid from one tank to the other to thereby alter thebalance and thus tilt the collector in the desired direction.
 15. Acollector as defined in claim 1 wherein the tracking mechanism iscontrolled by a servo mechanism fed by right and left light sensorsmounted either side of a vertical vane and aligned with the verticalaxis of the system.
 16. A collector as claimed in claim 1, in which theconcentration device is fabricated at least in part from plasticsmaterial.
 17. A collector as claimed claim 1, further including means toinhibit the formation of waves at the interface between the body ofliquid and the air.
 18. A collector as claimed in claim 17 in which themeans for inhibiting the formation of waves is substantially transparentto the solar radiation.
 19. A collector as claimed in claim 17 in whichthe means for inhibiting the formation of waves includes a floatingbarrier or a floating membrane.
 20. A collector as defined in claim 17in which the means for inhibiting the formation of waves includes afixed barrier.
 21. A collector as defined in claim 17, in which there aplurality of such means for inhibiting the formation of waves, each ofwhich is regularly spaced.
 22. A collector as claimed in claim 1, inwhich the liquid includes a component to suppress the growth of algae.23-24. (canceled)
 25. A collector for the collection of solar radiationcomprising: a. an energy conversion device including one or more of: (1)a photovoltaic cell, (2) a chemical reaction chamber, and (3) athermoelectric converter, wherein electromagnetic light energy isconverted to electrical or chemical energy, the energy conversion devicebeing substantially immersed in a body of liquid during such conversion;b. an energy concentration device including one or more of: (1) amirror, and (2) a lens, situated to concentrate electromagnetic lightenergy onto the energy conversion device, the energy concentrationdevice being at least partially immersed the body of liquid while theenergy conversion device converts electromagnetic light energy toelectrical or chemical energy; c. mechanical linkages reorienting thecollector with respect to the sun.
 26. A collector for the collection ofsolar radiation comprising: a. an energy conversion device including oneor more of: (1) a photovoltaic cell, (2) a chemical reaction chamber,and (3) a thermoelectric converter, wherein electromagnetic light energyis converted to electrical or chemical energy; b. an energyconcentration device including one or more of: (1) a mirror, and (2) alens, situated to concentrate electromagnetic light energy onto theenergy conversion device; c. a tracking mechanism including: (1) a tankmounted in connection with the energy concentration device, the tanktilting the energy concentration device when liquid is supplied to orremoved from the tank; (2) a pump in communication with the tank, thepump serving to supply liquid to, or remove liquid from, the tank.